Modern Ethernet Passive Optical Networks (EPON) use 10 Gigabit per second (10G) Ethernet transport, which is suitable to very high-speed data applications, as well as to converged system support (telephone, video, etc.). The unprecedented amount of bandwidth is directed toward, and arriving from a single entity, the Optical Network Unit (ONU). Normative numbers for ONUs in 10G EPON are 16, 32 or 64 and up to 256.
An EPON can be viewed as a distributed switch. An Optical Line Terminal (OLT) manages remotely the transmission of each ONU. The OLT and the ONUs exchange messages. In each cycle of such an exchange, the OLT grants a grant to each ONU, which is answered by a report message from the ONU. The ONU has a queue manager that prepares queue status information, which is transmitted using MPCP messages to the OLT to enable smart management. In other words, the ONU informs the OLT on its internal queues status. The OLT management is executed using a Dynamic Bandwidth Allocation (DBA) algorithm. An efficient algorithm is essential to guarantee Quality of Service (QoS), required to fulfill a Service Level Agreement (SLA). Methods for BW management are known, see e.g. US patent application No. 20080181248 by Haran et al.
The OLT transmits in downstream at full rate. The ONUs each transmit in upstream at an aggregate full rate where the transmission is divided among the ONUs in a time-division manner. In order to do proper BW management, each ONU reports the number of bytes that it has to transmit (referred to hereinafter as a “report value” or simply “report”). The OLT grants each ONU with a grant according to its report. This report-grant procedure is well known in the art. For 10G EPON the grants axe given by forward error corrected (FEC) codeword boundaries. The report values are given in units of time quanta (TQ). In 10G EPON (IEEE 802.3av standard) the normative values are 20 Bytes for one TQ and 216 Bytes without FEC parity and 248 Bytes with FEC parity for one FEC codeword.
The 10G EPON standard specifically requires a round-up of the reports to TQ units. In order not to request too little, the ONU rounds-up its request and sends the request as a REPORT to the OLT. For example, when rounding up to units of 20 Bytes, the ONU may request one excessive FEC codeword. The OLT needs to determine how many FEC codewords are needed by the ONU, and assigns the ONU a GRANT for N×FEC codewords. Note that while reports do not account for FEC parity, grants do. Therefore the OLT assigns N×248 Bytes and then it rounds this up to units of TQs (20 Bytes). When granted to the ONU, the excess FEC codeword may be wasted if the ONU has nothing to transmit. The OLT could have assigned this FEC codeword to another ONU and thus this upstream bandwidth would not have been wasted.
The following example illustrates the problem for one queue: assume the report net value is 201 Bytes. Due to the TQ round-up, the report value becomes 220 Bytes. Since round-up (220/216)=2, the OLT will assign two FEC codewords. The net grant (without FEC parity) will be 2*216=432 Bytes. The grant will thus have an excess of 231 Bytes instead of an excess of only 15 Bytes would the grant have been for one FEC codeword.
A prior art ONU-OLT report-grant mechanism is summarized in FIG. 1. In step 100 the GNU prepares a request to transmit data according to the occupancy (in bytes) of one or more transmission queues. This request is done via transmission of a report packet which reports the occupancy of each queue in units of TQ (also referred to as “report value”). “Occupancy” of a queue is defined herein to take into account any QoS functions (such as shaping and scheduling) applied. The report is provided in units of TQ. The report value is always rounded-up to TQ units in order not to request too little. In step 102, the OLT checks the reports from all ONUs and assigns respective grants to the ONUs in units of TQs. Since the transmissions use FEC, the OLT must make each grant long enough to as to accommodate a particular number of whole FEC codeword units, where the codewords will accommodate the report values and also FEC parity.
In view of the problem of the existing ONU-OLT report-grant mechanism in terms of wasted bandwidth, there is a need for, and it would be advantageous to have, an improved reporting mechanism for efficient upstream bandwidth utilization in 10G EPON.